The present invention relates to waveguide antennas and more particularly to rectangular and parabolic waveguide horn antennas.
Waveguide horn antennas are commonly used in wireless telecommunication systems to transmit and/or receive electromagnetic signals. FIG. 1 illustrates one such system in which four rectangular horn antennas are positioned 90.degree. apart. In this configuration, each antenna is designed to have a 90.degree. coverage area axially (.+-.45.degree.) to obtain a 360.degree. total coverage area, and a vertical coverage area defined by the sec.sup.2 .theta. beam pattern, known in the art.
One type of horn antenna used in this type of system is the conventional rectangular horn antenna 200, shown in FIG. 2A. The rectangular horn antenna has a body of length L and an output aperture 230 of height H and width W. The body also includes a source aperture 220 located at the back of the horn 210. The source aperture 220 may be a waveguide launch as shown or a coaxial launch as known in the art. Vertically or horizontally polarized waves my be launched from the rectangular horn antenna 200, depending upon the orientation of the applied signal.
The dimensions L, H, and W of the conventional antenna are dictated by several factors. The area of the output aperture 230 (H.times.W) determines the amount of antenna gain the horn will exhibit. The larger the output aperture 230, the more gain the antenna will exhibit.
The length of the horn antenna (L) is dictated by the requirement of phase coherent operation, i.e., signals must have a substantially planar wavefront when received at the source aperture 220 or transmitted from the output aperture 230. Because some of the received/transmitted signals will travel along the contours of the waveguide body and some along a direct (boresight) path, the horn must be long enough such that these two paths are substantially the same. These two paths converge as the horn length increases and diverge as the output aperture increases. Thus, as the output aperture area increases (to allow for more antenna gain), the horn length must also increase to maintain phase coherent operation.
FIGS. 2B-2D illustrate side views of three conventional rectangular horns having 15 dB, 20 dB, and 25 dB, of antenna gain, respectively. These figures indicate the degree to which the length of the conventional rectangular horn must by increased for higher gain operation. Each of the horns has an output aperture width of 1.07 cm.
As can be observed from the FIGS. 2B-2D, the horn length dramatically increases with increasing gain. The 20 dB gain horn is ten times as long as the 15 dB horn, and the 25 dB horn is approximately 100 times as long, measuring approximately 2 m long. While high gain, phase coherent horns are needed in telecommunication systems such as the base station shown in FIG. 1, their long length makes them extremely impractical.
What is needed is a compact waveguide antenna horn which provides high gain and phase coherent operation.